If I click on either minus or plus for radius the circle gets larger. If I type it the dimensions are changed to a multiple of 10. 
What am I doing wrong?

I'm not sure what you expect. Please compare these two minified versions (mm vs cm).

Minus increases it from 10 to 90 and plus from 10 to 110.
I expect minus to decrease it.
If I type 5 and hit enter it will turn to 50.
Same effect on your image.
I use the latest stable version of inkscape 1.4.2.

It works as expected here in both versions.
Inkscape 1.4.2 (f4327f4, 2025-05-13) MS Store version
Inkscape 1.4.2 (4b73df0, 2025-08-15) Stable bugfix version
 

Before proceeding further, I'd reset the preferences: https://inkscape.org/forums/questions/getting-back-to-square-one-and-starting-over-its-easy/

If that fails to resolve the issue, please describe step by step how the object is created , or better yet, share a screen recording of the process.

Also describe :

1. Describe your OS (your computer's Operating System: Mac/Windows/Linux/etc. & version).
2. Describe if Inkscape was newly installed, or upgraded from an earlier version.
3. Describe the version of Inkscape and the installer you used (the file you downloaded from the web: .msi/.exe/.zip/appimage/other, PPA/Snap/Flatpak, dmg/other ). 
4. Describe any special hardware being used, like tablet/stylus, external drives or multiple monitors.
5. Describe any helper/assistive programs or keyboard modifiers (macros, languages, etc.).

1. Windows 11
2. Inkscape 1.4.2 (f4327f4, 2025-05-13)
3. inkscape-1.4.2_2025-05-13_f4327f4-x64.exe
4. ultrawide monitor
5. no helper programs

I reinstalled it. I reset the defaults.Still the same problem.

Ah... I see you must be using the Object Properties to change the scale.  Yes, that is a bug.  Probably this: https://gitlab.com/inkscape/inkscape/-/issues/5307

The unified Object properties dialog is not fully developed. https://wiki.inkscape.org/wiki/Release_notes/1.4#Object_Attributes

I would use the ellipse tool to change the radius of the circle/ellipse.

workaround was setting scale from 0.1 to 1 and then selecting everything and resizing it to 0.1 the size.